This invention relates to gyroscopes (gyros) employed in inertial navigation systems, and more particularly to gyro stabilizing control circuitry adapted for particular two-degree-of-freedom dry tuned gyros used in strap-down navigation systems.
2. Description of the Prior Art
In the usual stable-platform inertial navigation system prior to departure of the vehicle from a known location and attitude, as when a space vehicle is on the ground awaiting launching, it is desirable to preserve the orientation of the gyro axes relative to the earth. For this reason, the gyros are electrically caged and the case of each gyro is also caged in order to constrain both the gyro and the accelerometer to a predetermined orientation relative to the earth which can be measured by suitable means relative to a fixed base line on the ground. At launching, the system is uncaged by freeing both gyro and case so that thereafter the gyro will preserve its initial spatial orientation irrespective of any maneuvers of the vehicle. The caging of the gyros is effected by means of electrical switches which are thrown at launch.
In a strap-down inertial navigation system, on the other hand, the inertial sensors, both rotational and translational, are secured to the air frame base. Both angular and translational data are processed in a high-speed guidance computer to continually update both rotational and translational references. The inertial references are stored analytically in a flight (or guidance) computer. The gyros are continually caged throughout the operation of the system. The only time the gyros of a strap-down navigation system are uncaged is when they are being spun up to synchronous operating speed. At such time, the switches completing the gyro caging loop are opened, but at all other times during operation of the system, the gyros are caged by means of an electrical caging loop.
The recently introduced two-degree-of-freedom dry tuned gyro (such as the Teledyne Model SDG-5) provides distinct advantages in inertial navigation systems as contrasted with previously known conventional gyros. By suitable orientation of three such units, complete redundancy can be provided. The disadvantage with such two-degree-of-freedom dry tuned gyro systems, when used for strap-down applications of relatively high bandwidth, is that the gyros normally require both cross axis and direct axis torquing together with complex shaping circuitry to provide a stable caging loop and achieve satisfactory noise rejection and good navigation accuracy.